def _rgb(self, im):
     h,w,s = im.as_rgba_str()
     
     rgba = fromstring(s, UInt8)
     rgba.shape = (h, w, 4)
     rgb = rgba[:,:,:3]
     return h, w, rgb.tostring()

 def _gray(self, im, rc=0.3, gc=0.59, bc=0.11):
     rgbat = im.as_rgba_str()
     rgba = fromstring(rgbat[2], UInt8)
     rgba.shape = (rgbat[0], rgbat[1], 4)
     rgba_f = rgba.astype(Float32)
     r = rgba_f[:,:,0]
     g = rgba_f[:,:,1]
     b = rgba_f[:,:,2]
     gray = (r*rc + g*gc + b*bc).astype(UInt8)
     return rgbat[0], rgbat[1], gray.tostring()